


convictioni

by armethaumaturgy



Category: Undertale (Video Game)
Genre: Baneberry, Ecto-Genitalia (Undertale), Edgeplay, Explicit Language, Explicit Sexual Content, Gentle Sex, Hand Jobs, Light Bondage, M/M, Overstimulation, Praise Kink, Safewords, Sex Magic, Size Difference, Underswap Sans/Horrorfell Sans, Vaginal Fingering, Vaginal Sex, cumflation, so they need help forming an ectobody. that, that thing where horror-aligned sanses dont have enough magic, think thats all. peace signs and fades out
Language: English
Status: Completed
Published: 2021-01-12
Updated: 2021-01-12
Packaged: 2021-03-17 04:54:28
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,488
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28719192
Author URL: https://archiveofourown.org/users/armethaumaturgy/pseuds/armethaumaturgy
Summary: "Too tight?" his lover asked him, looking down with nothing but love in his eyelights.Rust shook his head. "They're great."His wrists were cuffed behind his back and his shins were secured to a chair leg each. He knew it'd be easy enough to break either of them, or even the chair, if need be, but he also knew there wouldn't be need. It wasn't the first time he'd ended in such a position, and it probably wouldn't be the last.
Relationships: Sans/Sans (Undertale)
Kudos: 59





	convictioni

**Author's Note:**

> my brain is plagued by baneberry brainworms 24/7 :pensive: :pensive: smh

Rust flexed his arms, testing the bonds.

They didn't have much give, but they weren't biting into his bones, either. He used to wonder where Blue bought cuffs with fur lining from, but he refused to tell him.

"Too tight?" his lover asked him, looking down with nothing but love in his eyelights.

Rust shook his head. "They're great."

His wrists were cuffed behind his back and his shins were secured to a chair leg each. He knew it'd be easy enough to break either of them, or even the chair, if need be, but he also knew there wouldn't be need. It wasn't the first time he'd ended in such a position, and it probably wouldn't be the last.

Blue leaned forward and pressed a gentle kiss to Rust's teeth. Even sitting, Rust still had an inch or two over him. He melted into the contact, sockets falling closed as much as they could.

"Can you repeat the rules to me, Rusty?"

With Blue's breath fanning over his skull, Rust shivered, but then nodded nonetheless. "We're goin'… until y'come. Or um… until I can't. Can't move… and uh… I have ta listen ta ya."

Blue kissed him again, and he could just  _ feel  _ his beaming smile. "Great job, love. You didn't forget anything. I'm so proud. Can you also tell me your safeword?"

Rust's skull flushed a dull shade of red, the praise going straight to the little pool of magic nestled in his pelvis. "It's… it's red."

"Good, good. And the other one?"

"Other one…? Yellow… if I need ya ta slow down."

Blue made a very pleased noise in the back of his non-existent throat that had Rust smiling — almost beaming. He loved to please his lover. The whole safeword system was thought up by Blue, though he had his own words; blue and orange. Sometimes he still blurted out Rust's old name, in the heat of the moment, but Rust didn't mind. It was… cute, that Blue still saw him the same as he had ages ago.

"I'm going to start. Remember, you can use them anytime you need," Blue told him before stepping away from the chair. His hands came to rest on the bottommost of Rust's exposed ribs — but it wasn't like he wasn't fully exposed already. His touches were feather-light, but more than enough to pry a needy noise out of Rust when they passed over the numerous scars littered across the bones. It always seemed to catch him off guard just how sensitive they still were.

The coil of magic gathered in his pelvis wound tighter, fighting to manifest itself, but without much luck. Rust peered at Blue with a wide-blown pupil.

"Shortcake, sweetie..." he rumbled, "'m gonna… need a bit a'help."

Blue nodded and dipped one hand down to Rust's pelvis, ghosting over an iliac crest. Like an electrical shock, but so much more pleasant, magic sparked between them. It wasn't much, but enough for Rust's to react and snap into place between his legs and up to his ribcage. The dull color carried over from his blush, but on his stomach, just below where his ribs ended and ecto-flesh started, was a small splotch of blue, his lover's signature color.

He was cooed at, kissed again, and then Blue leaned down and pressed a kiss to that exact spot. His hands gripped Rust's sides, fingers sinking ever so softly into the plush, giving pseudo-flesh.

"You're so beautiful," he whispered, like a terribly important secret. Rust didn't agree, but he wasn't about to argue; Blue didn't like that.

The shorter skeleton nuzzled into the pudgy stomach, lathering the magic of it in kisses wherever he could reach. Rust shied from the attention, but he couldn't move enough for it to matter.

Blue and Stretch had made sure him and Rim ate their fills wherever it even  _ looked _ like they were hungry. It had taken months for his magic to stabilize, and it had been hard work, but his new pudge was one of the things he had to show for it. He was proud of it, he  _ was _ , but it reflected all the scars on the bones underneath. He couldn't understand how Blue could find  _ that  _ beautiful.

Small fingers traveled between his femurs and ran through the slick dripping from his entrance, and Rust jolted upwards, trying his best to push into the touch. As a reward, Blue pressed more firmly against the wet folds, rubbing small circles around the opening like he knew drove Rust wild.

They were both silent, up until the point Blue dipped two fingers into the giving ecto and Rust keened. His legs tried to spread further, to give Blue more space, but the cuffs prevented that.

"Oh, you are so soft," Blue cooed, pulling his fingers out only to slam them both in as far as they could go, "Always so tight and perfect."

The sensations, coupled with the praise, were a heady combination. Rust didn't know if that was Blue's intention or not.

"Pum'kin, if ya… keep goin', 'm gonna cum," he forced out. Blue, much to his immediate displeasure, pulled his fingers out.

He chuckled and his wet hand gripped Rust's thigh instead. He was leaning forward for another kiss. "Can you hold out for me?"

Rust, like always, nodded. So frantically, in fact, that he almost headbutted his lover. Blue just laughed more — but it was soft, amused, not mean in the slightest — and hushed him with a soft touch to his mandibles.

"Good boy," he said, punctuating it with another kiss.

His hand wrapped around Rust's cock, but maybe that was too generous; his fingers couldn't encircle the whole thing, not unless he used both hands. But his lover didn't complain. Sensitive as he was, even that touch had his spine arching as far off the chair as it could. Blue's touch stayed soft, his pumps short and slow, and every couple ones, he'd drag his palm over the weeping tip and make Rust cry out.

Blue was watching him oh so closely, and pulled away every time he got close, only to return to it once he'd calm down.

It was maddening.

It was perfect.

"Please," Rust begged, not even aware that he'd spoken aloud, or that he'd been doing so for the last three climaxes that he’d been denied. His babbles weren't even coherent words half the time, just jumbled versions of 'please,' intermingled with Blue's name and nicknames, strung together in a way that made sense to no one.

Blue was cooing back at him, meeting each word with a gentle shush, or a promise of how good he'd make Rust feel, and Rust didn't even know how to tell him he already  _ was _ . The small hand pumping him left  _ again _ , followed by a cry so loud and broken Rust would never admit it had come from him.

His little lover was saying something, but his acoustic meatuses were ringing, and he had no idea what it was. Then his fingers traced along the rim of his entrance and a couple dipped inside. There was enough slick for it to echo; Rust couldn't focus on anything else.

"Good job, darling," was the first thing he could understand Blue saying, but only because he was repeating it over and over, right into Rust's neck. "You did so well, Rusty. Just a second, okay? I'll make you feel good."

Rust gasped when the fingers withdrew, leaving him empty and aching, and he looked up at his lover through a sheen of tears that had gathered in his eyesockets, spilling over the jagged edges of his cracks. He tried to speak, but all that came out was a couple gasps and low moans.

Blue kissed him, thumbing the red tears from his sockets with gentle pressure. Rust's joints ached with excess magic, a rarity for him, and it took a long while for him to get down from the edge. Blue kept kissing him, gentle pecks and soft meshing of their tongues, until Rust started responding more consciously.

"You did amazing, love," Blue whispered against his teeth, voice full of reverence that Rust didn't deserve, but got anyway. "Do you know how many that was? Twelve," he told him, not waiting for an answer. "I'm so proud of you. You deserve a reward."

Even Rust himself was surprised at the number; he'd lost count at  _ two _ , for stars' sake. He preened under Blue's praise, grinning lopsidedly against the next kiss.

Blue pulled away, and he couldn't stop a disappointed whine of, "Shortcake..." from tumbling through his teeth. Blue just shushed him with a soft chuckle.

Rust watched as he started shedding his clothes; shorts first, then his jacket and boots, and finally, his black bodysuit. Each inch of exposed, unmarred bone sparked arousal all anew in him, saliva collecting on his tongue. Blue must've been doing it on purpose, if the glint in his eyelights was any indication, but Rust couldn't find it in himself to care.

Blue's arousal was obvious even before the bodysuit came off, the glow of his ecto-flesh bright enough to color his free bones. Rust could even see the wet patch left on the fabric as Blue set it aside.

"Ready, big boy?" Blue asked, clamoring his way onto Rust's lap. His spread legs were a perfect perch for him, like they'd been made for each other.

Rust was sure the question was rhetorical, but even if it wasn't, he didn't have enough time to respond. Blue gripped his shoulders to steady himself and sunk down onto Rust's cock in one smooth motion, his own cock leaving a trail of blue residue across Rust's stomach.

"Fuck, pum'kin!" Rust cried out, eyesockets squeezed closed at the onslaught of pleasure. Blue's passage was so wet, their combined slick creating a purple-hued puddle on the chair beneath them. "Oh, yer… yer so good..."

"Not as good as you," Blue told him, strained and quiet. He wiggled on Rust's lap, growing accustomed to the stretch.

Rust would beg to differ, but it was already a lost cause when it came to Blue. He was stubborn enough that they'd be going for hours, or until Rust forgot what they were even arguing about. But, to him, there was nothing like the feeling of Blue's ecto stretched around him, warm and snug. And when Blue unconsciously squeezed down on him, he could've sworn he saw stars.

Sometimes, with how easily Blue took him, it wasn’t hard to forget just how big the difference in their physiques was. Rust's color shone through Blue's stomach, almost all the way up to where his ribs started, the ecto-flesh visibly distended. If his hands had been free, Rust would've splayed one over the bump, just the way Blue liked. But they weren't, so he had to content himself with just watching as Blue gripped his hips with his knees and lifted himself, velvety walls dragging over Rust's length with the most exquisite of pressures.

Rust's claws dug into his palms, teeth grit when he rediscovered the restraints that prevented him from moving along. He would have felt ashamed of being brought to the brink with not even a full thrust, but after being strung up for so long, all he yearned for was a release.

"H-Honey," he growled, staring at his lover with half-lidded sockets. It probably looked weird, with his cracks, but Blue had never mentioned it. "Sweet— sweetpea… 'm… 'm close."

Blue let gravity aid him in bouncing back down, a loud whine leaving him as Rust hit the deepest part of his passage. He felt so full, but they both knew he could take more, so much more. He wrapped his arms around Rust's shoulders, phalanges trailing feather-light across the dulled edges of his old injury.

"You don't need to hold back anymore," he said, setting a slow pace. "You did wonderfully, you deserve to come, love. Come on, give me all you've got."

Rust didn't need to be told twice; almost like a dam breaking, magic surged within him, the coil in his soul snapping. He came with a cry of Blue's name, hips stuttering just off the chair as his cum filled Blue's already-full passage. The ecto did its best to accommodate the extra volume, but there was only as far as it could physically stretch, and a flood of red magic leaked around Rust's cock to soak their laps and the chair and, at this point, the floor as well.

Blue shivered against Rust's ribcage, walls spasming around him like they intended to coax every last drop of magic out of him.

"Stars, yes,  _ yes _ ," Blue was hissing, resuming his slow bouncing with a blissed expression that Rust wished he could sear into his memory forever. "A-Are you okay to keep going? Or do you—" Blue cut himself off with a moan, apparently finding a good spot, or maybe his cock had dragged against Rust's stomach in just the right way. Rust took the opportunity to capture him in a kiss, teeth clacking together, though neither of them seemed to be bothered by it.

"Gon' need 'nother spark, cupcake," Rust said, breathless. He was straining to keep his ecto-flesh formed, and Blue must have noticed as well. He stilled long enough to nod and press a hand between them, over one of Rust's floating ribs.

Embarrassingly, just the spark of magic flicking between them to stabilize his body was enough for another gush of fluid to drench the chair, which would definitely be stained after this.

Blue lifted the hand up to his face and cradled the side of it, as soft as ever.

"Mweh heh," he chuckled, "we match now."

Rust had to look down to see what Blue was talking about; the patch of blue magic on his stomach was bigger now, much bigger, and it did resemble the way he could see his own cock through Blue's stomach. He exhaled a long groan.

"Love, can you give me a color?"

It took another moment to process the request, and when Rust did, he could only shake his head. "'m… 'm good, sweetpea," he mumbled.

Blue kissed him, frantic and hard, and lifted his hips only to drop down so hard their thighs smacked together.

"Good," the smaller skeleton sighed out, one hand leaving Rust's shoulder to worm its way between them and wrap around his cock, only adding more wet noises to the cacophony. "Don't hold back anymore. Cum if you want to. I'll give you— I'll give you as much magic as you can take. So…" He held Rust's gaze and licked his teeth, a trail of their combined saliva running down the corner of his lopsided grin, "So give me all of it, okay?"

And then he started riding him in earnest, and Rust couldn't deny his baby Blue anything he ever wanted.

**Author's Note:**

> you can talk to me on [tumblr](https://armethaumaturgy.tumblr.com/) or [twitter](https://twitter.com/esqers)


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